Preheat the oven to 325 degrees F.
Bring a medium pot of water to boil for the onions. Salt the water and boil the onions for 5 minutes. Then drain the onions and cool to handle. Peel the onions by squishing the root end out of the skins.
Bring the beef to room temperature and pat dry with paper towels. Sprinkle with salt and pepper. Heat the oil over medium-high heat in a large Dutch oven and add the beef in batches, browning until deeply caramelized all over. Repeat until all the beef has been browned, adding more oil when necessary. Remove the browned meat to a plate, covered with foil and reserve for later use.
Reduce the heat to medium-low and add the butter to the same pot used for browning the meat. When the butter foams, add the onions to the pot. Season with salt and pepper, and add in the thyme, allspice, cloves and some nutmeg. Cook until light caramel in color, 20 to 25 minutes, stirring occasionally. Drizzle in the honey. Then add the sherry to deglaze the pan. Next, add the stock and the browned beef. Cover and roast in the oven for 1 hour 15 minutes. Add the pears and stir to combine. Roast until the beef is tender, 45 minutes to 1 hour more. Cool and store in the refrigerator for a make-ahead meal.
When ready to serve, reheat the potted beef in a 325 degrees F oven or on the stove-top, covered over medium-low heat.
Stir up the sour cream with the chives, heavy cream and horseradish. Season with salt and pepper. Serve atop the beef and onions with pears.